A distributed amplifier has a unique architecture with the inherent benefit of supporting very broad band operation, when compared with more conventional amplifier architectures. As illustrated in FIG. 1, a typical distributed amplifier 100 includes a plurality of periodically distributed amplifier paths (or “feeding branches”) coupled between an input transmission line 110 and an output collection line 150. More specifically, each feeding branch includes a power amplifier 131-134 with an input coupled to the input transmission line 110 at a feeding node 121-124, and with an output coupled to the output collection line 150 at a tap node 151-154.
The input transmission line 110 is configured to receive a radio frequency (RF) input signal 102 at an input end of the transmission line 110. The feeding nodes 121-124 are spaced apart (or “distributed”) along a length of the input transmission line 110, and each input transmission line segment (i.e., a portion of transmission line between consecutive feeding nodes 121-124) is configured to impart a delay to the input RF signal as the signal propagates through the segment. Accordingly, the feeding node 124 closest to the input end receives the RF signal after a relatively short delay, and the feeding node 121 farthest from the input end receives the RF signal after a relatively long delay.
The output collection line 150 is configured to combine amplified RF signals produced by the feeding branches, and to produce an output RF signal 104 at an output end of the output collection line 150. Similar to the feeding nodes 121-124, the tap nodes 151-154 also are spaced apart along a length of the output collection line 150, and each output collection line segment (i.e., a portion of transmission line between consecutive tap nodes 151-154) is configured to impart a delay to the RF signal as the signal propagates through the output collection line segment. Accordingly, the amplified RF signal produced by the amplifier path with a tap node 154 farthest from the output end is produced at the output end after a relatively long delay through the output collection line 150, and the amplified RF signal produced by the amplifier path with a tap node 151 closest to the output end is produced at the output end after a relatively short delay through the output collection line 150.
The RF signals propagating toward the output end of the output collection line 150 combine at each of the tap nodes 151-153 with an amplified RF signal received from a corresponding feeding branch. Accordingly, the tap nodes 151-153 function as combining or summing nodes, and from tap node 154 to tap node 151, the RF energy continues to build along the output collection line 150. The positions of the feeding nodes 121-124 and the positions of the tap nodes 151-154 along the input transmission line 110 and the output collection line 150, respectively, are selected so that the amplified RF signal produced by each feeding branch is combined in phase with the preceding signal propagating on the output line at the tap nodes 151-153. In other words, the total delay through the amplifier 100 that is experienced by any component of the RF signal is the same between the input end of the input transmission line 110 and the output end of the output collection line 150, regardless of which feeding branch the signal component traveled through.
Further, the driving point impedances at the tap nodes 151-154 may have very broad band constant values when the output collection line 150 has been appropriately optimized. To achieve the desired broad band loads along the output collection line 150, the characteristic impedances of the output collection line segments may be designed to step in a particular geometric sequence. An output collection line 150 designed in such a manner is referred to as a “tapered” transmission line.
A distributed amplifier performs at its best when every aspect is optimized. This includes correctly managing signal amplitude and phase through the amplifier, judiciously choosing interim impedance levels for the segments of the output collection line 150, and correctly sizing the amplifier's active devices (e.g., the power transistors of the feeding branch amplifiers 131-134). However, such optimizations often are not enough to ensure desired performance in conventionally-designed distributed amplifiers, particularly in higher power distributed amplifiers intended for wideband operation.